A winter snowstorm moving through parts of the South is expected to dump some snow in the D.C. region. Capital Weather Gang's Jason Samenow tells us what areas could get the steadiest snow and when. (Claritza Jimenez,Jason Samenow/The Washington Post)

A major winter storm passing south and east of Washington will come close to enough to spread snow over parts of the region. Heavy snow is possible in Southern Maryland and the southern and central Delmarva, while lighter snows are likely to skirt the immediate D.C. area.

The D.C. area sits on the very northwest periphery of this storm’s predicted precipitation which is always the most difficult area to predict amounts. Very small changes in the storm track and strength could mean substantial changes to the snowfall forecast, downward or upward.

Snow is likely to develop over the region from south to north during the pre-dawn hours on Saturday. Temperatures will be cold enough — below freezing area-wide — that snow will stick fairly quickly. Untreated roads are likely to become slick and hazardous, especially in areas south and southeast of the Beltway and Interstate 95.

Warnings and advisories issued by National Weather Service.

By sunrise, snow is possible over much of the metro region, although it may be spotty and light in our far north and western areas while moderate-to-heavy in Southern Maryland (Calvert and St. Mary’s counties).

Through the late morning, the most likely scenario is that light snow and flurries — falling intermittently — dust our northern areas, while steady, accumulating snow pastes our southern areas.

GFS model simulation of snowfall to affect Mid-Atlantic Saturday.

During the afternoon, snow will quickly taper off from northwest to southeast. In fact, the snow may already be over west of Interstate 95 by noon, and should be ending in Southern Maryland by 3 or 4 p.m. unless the storm slows down or turns more to the north.

Amounts, which are still subject to change, are likely to vary significantly across the region. The same places that received accumulating snow Thursday night, in western Loudoun and Frederick counties, may just get a dusting or so — and no snow is a possibility. Meanwhile, 4 to 8 inches are possible in southern Calvert and St. Mary’s counties.

Inside the Beltway and near Interstate 95, snow amounts should range from one to three inches, although these totals could increase if the storm tracks closer to the region or decrease to less than an inch if the storm shifts somewhat east.

In addition to the snow, locations near the Chesapeake Bay may deal with some gusty winds — up to 25 mph — blowing the snow around a bit.

Temperatures in the 20s mean the snow will tend to be powdery and stack up quickly where it falls steadily.

Using Capital Weather Gang’s new Winter Storm Impact Scale, we rate this storm a Category 1 “nuisance” storm for much of the close-in D.C. and Baltimore areas, where one to three inches of snow is expected. The amount of snow projected is not much, but because of cold temperatures before, during, and after the storm – any snow that falls will stick and cause slick roads.

If snow amounts exceed two inches, this storm will produce impacts more consistent with a more disruptive Category 2 storm. However, if snowfall ends up just being a dusting, then it wouldn’t merit a rating at all.

Stafford, Charles, and northern Calvert counties

Storm impacts in this zone, in which 3-6 inches of snow are predicted, is rated a Category 2. Snow falling on cold roads is likely to cause slick roads throughout this region. Cold weather will persist after the storm, meaning the snow will stick around.

Southern Calvert and St. Mary’s counties

In this zone, where we expect 4 to 8 inches, this is a significant winter storm. The combination of heavy snow at times, cold temperatures, and wind, will make for widespread hazardous travel Saturday.

Model discussion

The models continue to flip from one solution to another, making any snow accumulation prediction extremely challenging.

A comparison of the last two NAM model runs (below) provides insight into how differences in the track and intensity of the system can impact our snow totals. Today’s run (top) has the low a little farther off the coast and it keeps the precipitation shield farther to the south and east, especially the heavier (darker blue shades) which the earlier model run spread back into Washington (bottom).

(WeatherBell.com)

The earlier run would have supported a couple of inches of snow while today’s run argues for a dusting to maybe an inch. (The light green on the western edge, indicating rain, is misleading and with such cold temperature would be snow. )

Today’s GFS model trended in the opposite direction of today’s NAM model, increasing snow amounts in the metro area. Only small differences in the forecasts of the low pressure system and associated upper level energy lead to differences in how far north the snow spreads. Last night’s version of the GFS (top) argued for little or no snow, while today’s forecast (bottom) supports an inch or two into the city and upward of four inches across southern Calvert and all of St. Mary’s counties.

(WeatherBell.com)

Today’s GFS model is very much in line with last night’s European model forecast.

If you look at the upper level (500 millibar) and surface forecast maps for the last two runs of the GFS model (below) you can get a sense of how small differences aloft and at the surface can change the precipitation shield.

(StormVistaWxModels.com)

Note on the upper level charts the green area labeled 540 and how it dips a little farther south on today’s run (top) compared to last night’s (bottom). The upper system is a little stronger.

Now look at the pressures along the Southeast coast, indicated by the black contours, and note that the low is a little stronger on today’s run (top) than on the previous one (bottom).

Additional small changes in these upper level and surface features could either boost our snow totals or decrease them.

Our region is on the edge of the storm where it is really easy for the forecast to bust. We urge readers to monitor forecast updates and be aware that the most likely forecast may not be the forecast that turns out to be correct.

Model snow forecasts

Of the various models, whose snowfall forecasts we present below, we’re leaning toward the GFS, European and Canadian models, which predict more snow, over the forecast from the NAM, which predicts less — although it could end up being right.